The present invention relates to signal enhancement systems and, more particularly, to acoustic arrays used on underwater surfaces.
Modern sonar arrays used on the exterior surfaces of submarine hulls often include a layer of acoustic isolation baffles positioned adjacent to the hull for reducing the radiation of shipboard noise. These arrays also include an outer layer containing a plurality of hydrophone units, and an intermediate sound conditioning layer secured to the isolation baffle for improving the quality of the incoming signals.
The acoustic isolation baffles attenuate, absorb and reflect shipboard noise by means which may include, for example, one or more layers of a composite material such as "Corprene", in which cork particles are embedded in a neoprene matrix; and gas and/or air filled multicellular materials such as foam rubbers. However, these types of sound dissipation means often share a common drawback in being pressure and frequency dependent. For example, the impedance of light materials such as cork and cellular substances changes as the materials are compressed so that the acoustic absorption efficiency is likewise changed.
The conditioning layer is used to provide a mass reactance which coherently reflects the incoming signals so that the reflected signals reinforce the incident acoustic signals. These conditioning layers normally include a "backing plate" of steel or some similar material which has an impedance higher than water and a thickness of about one-quarter of the wave length of the predominant, incident acoustic frequency in the backing plate material. At this thickness the hydrophones will have maximum efficiency and sensitivity for incoming acoustic signals of the preferred frequency. However, for comparatively low acoustic design frequencies, the requisite thickness and weight of the quater-wave thick plate can become prohibitive. For example, a quarter-wave steel plate for a design frequency of 2.4 KHz would have a thickness of about 52 cm. Thus at low frequencies the great weight and bulk of solid "backing plate" arrangements becomes prohibitive.